Aluminum alloy sheet

ABSTRACT

The present invention relates to an Al—Mg—Si sheet which contains, in terms of mass %, 0.3-1.0% Mg, 0.5-1.5% Si, 0.005-0.2% Sn, 0.02-1.0% Fe, and 0.02-0.6% Mn, with the remainder comprising Al and unavoidable impurities, characterized by having a structure wherein compounds that are detected with an SEM having a magnification of 500 diameters and are identified with an X-ray spectrometer include Sn compounds which contain Mn and Fe and which have an Sn content of 1.0 mass % or higher and a diameter of 0.3-20 μm in terms of equivalent circular diameter, the average number density of the Sn compounds being 500-3,000 /mm 2 , and wherein the length of the boundary between each Sn compound and the aluminum matrix is in the range of 3-20/mm on average in terms of value obtained by dividing the total peripheral length of the Sn compound grain by the area thereof determined with the SEM. This aluminum alloy sheet satisfies the formability after natural aging at room temperature and bake hardenability which are required of automotive outer panels, and has excellent filiform rust resistance.

TECHNICAL FIELD

The present invention relates to an Al—Mg—Si alloy sheet, and especiallyrelates to an aluminum alloy sheet excellent in formability, BH responseand corrosion resistance. The term “aluminum alloy sheet” used in thepresent invention means an aluminum alloy sheet that is a rolled sheet,such as a hot-rolled sheet or cold-rolled sheet, and that has undergonerefining, such as a solution heat treatment and a quenching treatment,and has not undergone a bake hardening treatment. Hereinafter, aluminumis referred to also as Al.

Background Art

In recent years, the social request for weight reduction in vehiclesincluding automobiles is increasing more and more due to considerationsto the global environment or the like. In order to meet the request,aluminum alloy materials which are excellent in terms of formability andbake hardenability and are more lightweight are coming to beincreasingly used as materials for automotive panels, in particular,large body panels such as hood, door and roof (outer panels and innerpanels), in place of steel materials such as steel sheets.

Among those large body panel structures of automobiles, as outer panels(outer sheets), such as hoods, fenders, doors, roofs, and trunk lids,use of Al—Mg—Si-based AA or

JIS 6000-series aluminum alloy sheets which are thin high-strengthaluminum alloy sheets is being investigated.

As is known well, the automotive outer panel is produced by subjecting a6000-series aluminum alloy sheet as a material to combined formings suchas stretch forming in press forming and bending forming. For example, inthe case of a large outer panel such as a hood or door, the shape of aformed product as the outer panel is imparted by press forming such asstretching and then the peripheral edge part of this outer panel issubjected to hem work (hemming) to form a flat hem or the like andthereby joining with an inner panel is performed. Thus, a panelstructure is obtained.

The 6000-series aluminum alloy sheets have the advantage of havingexcellent BH response (bake hardenability) but have room-temperatureaging properties. There has hence been a problem in that they, when heldat room temperature after a solution quenching treatment, undergo agehardening and increase in strength, thereby deteriorating in formabilityinto panels. Moreover, in the case where such room-temperature aging isgreat, there is also a problem that the BH response deteriorates and theheating during an artificial aging (hardening) treatment at a relativelylow temperature, such as a paint baking treatment of the panel afterbeing formed, does not improve the proof stress to such a degree thatthe panel comes to have the required strength.

A large number of metallurgical measures for overcoming those have beenproposed for far. One of these is a method in which Sn is positivelyadded to a 6000-series aluminum alloy sheet in order to inhibitroom-temperature age hardening and improve the BH response. For example,Patent Document 1 proposes a method in which Sn is added in anappropriate amount and a solution heat treatment and subsequentpreliminary aging are performed to thereby obtain both of suppressedroom-temperature age hardening and BH response. Patent Document 2proposes a method in which Sn and Cu, which improves formability, areadded to a 6000-series aluminum alloy sheet to improve formability, bakehardenability and corrosion resistance.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-09-249950

Patent Document 2: JP-A-10-226894

SUMMARY OF THE INVENTION Problem that the Invention is to Solve

In those conventional 6000-series aluminum alloy sheets to which Sn hasbeen positively added, there has still been room for improvement forachieving, as materials for automotive outer panels, combinedsatisfactory formability and high BH response after long-termroom-temperature aging with various properties including excellentfiliform corrosion resistance.

For example, improvements in filiform corrosion resistance are essentialas automotive outer panels (panels for outside use). Although used afterbeing painted, automotive outer panels are exposed to corrosiveenvironments (under-paint-film corrosive environments) involvingseawater, brine, or the like as environments in which the automobilesrun. There is hence a problem in that threadlike rust called filiformrust generates from precipitates or inclusions as origins and grows onthe surface of the aluminum alloy sheet beneath the paint film,resulting in a decrease in the strength of the member and an appearancefailure. It is therefore necessary that Sn-added Al—Mg—Si alloy sheetsfor use as automotive outer panels should have excellent filiformcorrosion resistance.

In 6000-series aluminum alloy sheets also, various techniques forimproving the compositions or microstructures of the matrixes in orderto improve the filiform corrosion resistance have been proposed so far.However, the case where Sn has been added shows a metallurgical behaviordifferent from that in the case where no Sn has been added, and it hasbeen uncertain as to whether the above-described conventional techniquesfor improving the matrixes are effective, as expected, also in the casewhere Sn has been added. Consequently, for improving the filiformcorrosion resistance of a 6000-series aluminum alloy sheet to which Snhas been positively added, together with the other properties mentionedabove, such as formability and BH response, it is necessary to pursue adistinctive measure for improving Sn-added 6000-series aluminum alloysheets.

With respect to the formability also, the properties required of6000-series aluminum alloy sheets as materials for automotive outerpanels tend to become severer increasingly. Automotive outer panels arerequired to attain strain-free, beautiful curved-surface configurationsand produce character lines just as designed. This is a problemattributable to the peculiar designs of outer panels. Recessed portionshaving given depths (protrudent portions, embossed portions) forattaching devices or members, such as knob mount bases, lamp mountbases, and license (number plate) mount bases, or for drawing wheelarches are partly provided to outer panels.

In the cases when such a recessed portion is press-formed together withconsecutive curved surfaces which surround the recessed shape, facestrains are prone to occur in 6000-series aluminum alloy sheets, whichhave poorer formability than steel sheets, and it is difficult to attainthe strain-free, beautiful curved-surface configuration and characterline. Consequently, it is essential for 6000-series aluminum alloysheets that the occurrence of such face strains during forming thereofinto automotive outer panels should be inhibited. The problem of suchface strains is not a problem only for those recessed portions(protrudent portions) but a problem common to automotive panels whichpartly have a recessed portion (protrudent portion) that may suffer aface strain, such as the saddle-shaped portion of a door outer panel,the vertical wall portion of a front fender, the window corner portionof a rear fender, the character-line termination portions of a trunk lidor hood outer panel, and the root portion of a rear fender pillar.

From the standpoint of inhibiting the occurrence of the face strains toovercome the problem, it is desirable that a 6000-series aluminum alloysheet (which has undergone room-temperature aging after production)should have a reduced 0.2% proof stress when press-formed. However, inthe cases when the proof stress in press forming has been reduced, it israther difficult to obtain a high proof stress after a bake hardeningtreatment (bake hardening).

The present invention has been achieved in order to overcome suchproblem. An object thereof is to provide an Sn-containing 6000-seriesaluminum alloy sheet which satisfies the requirements for use asautomotive outer panels, concerning formability and BH response afterroom-temperature aging, and which further has improved filiformcorrosion resistance.

Means for Solving the Problem

For achieving the object, the gist of the aluminum alloy sheet of thepresent invention is an Al—Mg—Si alloy sheet containing, in terms ofmass %, 0.3-1.0% of Mg, 0.5-1.5% of Si, 0.005-0.2% of Sn, 0.02-1.0% ofFe, and 0.02-0.6% of Mn, with the remainder being Al and unavoidableimpurities, the aluminum alloy sheet having a microstructure in which,among compounds examined with an SEM with a magnification of 500 timesand identified with an X-ray spectrometer, an Sn compound containing Mnand Fe, having an Sn content of 1.0 mass % or higher, and having anequivalent circular diameter in a range of 0.3-20 μm, has an averagenumber density in a range of 500-3,000 counts/mm², and a boundarybetween the Sn compound and an aluminum matrix has a length in a rangeof 3-20 /mm on average in terms of a value obtained by dividing a totalperipheral length of the Sn compound by an area examined with the SEM.

Effects of the Invention

In the microstructure of the 6000-series aluminum alloy sheet, the Snhas an action to capture (trap) atomic holes in a room-temperaturestate. Due to this action of Sn, the room-temperature diffusion of Mgand Si is inhibited to suppress room-temperature aging (hardening) andinhibit the strength from increasing. Thus the effect of improving thepress formability including hem workability, drawability, and punchstretch formability during the forming of the sheet into panels isbrought about. Meanwhile, during an artificial aging treatment of thepanels, such as a paint baking treatment, the Sn releases the capturedholes and hence has the effect of in tern enhancing the diffusion of Mgand Si to heighten the BH response.

However, the present inventors have found that the Sn's effect ofcapturing and releasing atomic holes is exhibited only when the Sn formsa solid solution in the matrix. However, the amount in which Sn forms asolid solution in the matrix is so small that even when the additionamount of Sn is reduced to or below a theoretical solute amount inordinary sheet production processes, a large proportion thereof does notform a solid solution and undesirably crystallizes out or precipitatesas compounds. The Sn which has thus crystallized out or precipitated ascompounds does not have the effect of capturing and releasing atomicholes, although it has the effect of improving the filiform corrosionresistance which will be described later.

Because of this, in the present invention, the present inventors haveventured to reconsider sheet production processes and contrivedproduction conditions concerning, for example, soaking treatment tocontrol the number density of Sn-containing compounds having a specificcomposition and a specific size, thereby controlling a balance betweenthe formation of solid solution and precipitation of the Sn containedand ensuring a solute Sn amount, as will be described later. Thus, agehardening is suppressed by producing both the solute Sn's effect ofcapturing and releasing atomic holes and the effect of the presence ofthe Sn compounds having the specific composition and size, therebyimproving the formability and the BH response. Specifically, the sheetproduced is made to have the following properties after room-temperatureaging: the proof stress during press forming into automotive outerpanels (before bake finish) is 110 MPa or less; the hem workability is2.0 or less in terms of the criteria which will be described later inExamples; and the artificial-aging hardening amount (BH response) as anautomotive outer panel under bake finish conditions of 185° C.×20 min is100 MPa or greater.

Meanwhile, in the present invention, precipitates or crystals are formedso that boundaries between the Sn compounds having the specificcomposition and size and the aluminum matrix are large (long) in orderto improve the filiform corrosion resistance. Thus, boundaries betweencompounds containing no Sn and the matrix can be made small (short).Consequently, a 6000-series aluminum alloy sheet which combinessatisfactory filiform corrosion resistance with formability and BHresponse can be provided.

MODES FOR CARRYING OUT THE INVENTION

Modes for carrying out the present invention will be explained below indetail with respect to each requirement.

(Chemical Component Composition)

First, the chemical component composition of the Al—Mg—Si (hereinafterreferred to also as 6000-series) alloy sheet of the present invention isexplained below. As sheets for automotive outer panels, 6000-seriesaluminum alloy sheets to which the present invention relates arerequired to have various properties including excellent formability andBH response after room-temperature aging and filiform corrosionresistance.

It is preferable that the sheets should have the following propertieswhich are necessary for satisfying those requirements: as properties ofthe sheets which are produced and then have undergone refining, e.g.,T6, and subsequently undergone 30-day room-temperature aging, a proofstress during press forming into automotive outer panels (before bakefinish) is 110 MPa or less and a hem workability, in terms of thecriteria which will be described later in Examples, is 2.0 or less; andas an automotive outer panel, an artificial-aging hardening amount (BHresponse) under bake finish conditions of 185° C.×20 min is 100 MPa orgreater.

With respect to requirements concerning alloy composition for satisfyingthose preferred sheet properties, the aluminum alloy sheet has aspecific composition among 6000-series, containing, in terms of mass %,0.3-1.0% of Mg, 0.5-1.5% of Si, 0.005-0.2% of Sn, 0.02-1.0% of Fe, and0.02-0.6% of Mn, with the remainder being Al and unavoidable impurities.All indications by % of the each element content mean mass %. In thisdescription, percentage on mass basis (mass %) is the same as percentageon weight basis (wt %). With respect to the content of a chemicalcomponent, there are cases where “X % or less (exclusive of 0%)” isexpressed by “more than 0% and X % or less”.

Among ones having the above alloy composition, preferred is a6000-series aluminum alloy sheet with excess Si in which the mass ratioof Si to Mg, Si/Mg, is 1 or greater and which has better BH response.

Elements other than Mg, Si, Sn, Fe, and Mn as the alloy composition areunavoidable impurities, and are regulated to contents (permissibleamounts) on element levels according to the AA or JIS standards, etc.Namely, in the present invention also, in the cases where not onlyhigh-purity Al base metal but also 6000-series alloys, other aluminumalloy scrap materials, low-purity Al base metal, and the like are usedin large quantities as melted raw materials for the alloy, from thestandpoint of resource recycling, other elements other than Mg, Si, Sn,Fe, and Mn are inevitably included in substantial amounts. Sincerefining performed for intentionally diminishing these elements itselfleads to an increase in cost, it is necessary to accept some degree ofinclusion so long as the inclusion does not inhibit the object oreffects of the present invention.

Specifically, there may be contained one kind or two or more kindsselected from the group consisting of 0.4% or less (exclusive of 0%) ofCr, 0.3% or less (exclusive of 0%) of Zr, 0.3% or less (exclusive of 0%)of V, 0.1% or less (exclusive of 0%) of Ti, 0.4% or less (exclusive of0%) of Cu, 0.2% or less (exclusive of 0%) of Ag, and 1.0% or less(exclusive of 0%) of Zn, in terms of mass %.

The content range and the purposes of each element or permissible amountthereof in the 6000-series aluminum alloy are explained below in order.

Si: 0.5-1.5%

Si, as a major element, is an essential element for contributing tosolid-solution strengthening, and for forming Mg—Si precipitates whichcontribute to an improvement in strength, during an artificial agingtreatment such as a paint baking treatment, thus exhibiting agehardenability and thereby obtaining the strength (proof stress) requiredof automotive outer panels. From the standpoint of exhibiting excellentage hardenability in a paint baking treatment after forming into panels,it is preferable that the 6000-series aluminum alloy is made to have acomposition which has an Si/Mg mass ratio of 1.0 or greater and in whichSi has been incorporated in a larger amount, relative to Mg, than in theso-called excess Si type. In the case where the content of Si is toolow, Mg—Si precipitates are yielded in an insufficient amount, resultingin a considerable decrease in BH response.

Meanwhile, in the case where the content of Si is too high, coarsecrystals and precipitates are formed within grains and at grainboundaries, resulting in considerable decreases in bendability andfiliform corrosion resistance. Consequently, the Si is in the range of0.5-1.5%. A more preferred lower limit thereof is 0.6%, and a morepreferred upper limit thereof is 1.4%.

Mg: 0.3-1.0%

Mg also, as a major element, is an essential element for contributing tosolid-solution strengthening, and for forming Mg—Si precipitates whichcontribute to an improvement in strength, during an artificial agingtreatment such as a paint baking treatment, thus exhibiting agehardenability and thereby obtaining the proof stress required of panels.In the case where the content of Mg is too low, Mg—Si precipitates areyielded in an insufficient amount, resulting in a considerable decreasein BH response. Consequently, the proof stress required of panels is notobtained. Meanwhile, in the case where the content of Mg is too high,coarse crystals and precipitates are formed, resulting in a considerabledecrease in bendability. Consequently, the content of Mg is in the rangeof 0.3-1.0%. A more preferred lower limit thereof is 0.4%, and a morepreferred upper limit thereof is 0.8%.

Fe: 0.02-1.0%

Fe is an element necessary for yielding, in a specific number density,Sn—containing compounds of a specific size which are specified in thepresent invention, together with Al and other elements including Si, Mnand Sn during a soaking treatment and hot rolling. In the case where thecontent thereof is too low, the specific Sn-containing compounds areyielded in so small an amount that the boundaries between the specificSn-containing compounds and the matrix become small (short), resultingin a decrease in the effect of improving filiform corrosion resistance.Meanwhile, in the case where the Fe content is too high, the specificSn-containing compounds are yielded in too large an amount within grainsand at grain boundaries, resulting in deteriorations in formability suchas hem workability and in filiform corrosion resistance.

Mn: 0.02-0.6%

Like the Fe, Mn is an element necessary for yielding, in a specificnumber density, Sn-containing compounds of a specific size which arespecified in the present invention, together with Al and other elementsincluding Si, Fe and Sn during a soaking treatment and hot rolling. Inthe case where the content thereof is too low, the specificSn-containing compounds are yielded in so small an amount that theboundaries between the specific Sn-containing compounds and the matrixbecomes small (short), resulting in a decrease in the effect ofimproving filiform corrosion resistance. Meanwhile, in the case wherethe Mn content is too high, the specific Sn-containing compounds areyielded in too large an amount within grains and at grain boundaries,resulting in deteriorations in formability such as hem workability andin filiform corrosion resistance.

Sn: 0.005-0.2%

Sn is an essential element and in a solid-solution state at roomtemperature, it has the effects of capturing atomic holes to therebyinhibit room-temperature diffusion of Mg and Si and inhibit aroom-temperature increase in strength (room-temperature age hardening)from occurring over a prolonged period, and of improving the pressformability, in particular hem workability, of the sheet when the sheetwhich has undergone room-temperature aging is press-formed into panels.Meanwhile, during an artificial aging treatment of the formed panels,such as a paint baking treatment, the Sn releases the captured holes andhence in turn enhances the diffusion of Mg and Si, thereby enhancing theBH response.

These effects of Sn are exhibited only when the Sn forms a solidsolution. In the case where the content of Sn is too low, a decrease insolute Sn amount results and holes cannot be sufficiently trapped,making it impossible to produce the Sn's effect of inhibitingroom-temperature age hardening. As a result, not only theroom-temperature increase in strength cannot be inhibited, resulting inan increase in proof stress and a deterioration in hem workability, butalso Mg—Si precipitates are prone to be yielded in a reduced amountduring a BH treatment, resulting in a decrease in BH response.

In the present invention, other than the Sn being made to form a solidsolution, Sn is caused, in a certain amount, to precipitate orcrystallize out as Sn-containing compounds to improve filiform corrosionresistance. However, in the case where the content of Sn is too low, theamount of Sn-containing compounds also is decreased.

Consequently, among the compounds containing Mn and Fe, the averagenumber density of compounds which have a content of Sn of 1.0 mass % orhigher and an equivalent circular diameter in the range of 0.3-20 μm isinsufficient. As a result, the length of the boundaries between thesecompounds and the aluminum matrix also is insufficient, making itimpossible to improve the filiform corrosion resistance.

It is, however, noted that even when the content of Sn is increasedexcessively, solute Sn amount does not increase since there is a limiton the solid-solution amount. In addition, in the case where the contentof Sn is too high, Sn segregates at grain boundaries and this iscausative of intergranular cracks. As a result, cracks are prone tooccur during hot rolling in sheet production steps.

Consequently, the content of Sn is in the range of 0.005-0.2%. A morepreferred lower limit thereof is 0.01%, and a more preferred upper limitthereof is 0.18%.

(Microstructure)

Next, the microstructure of the 6000-series aluminum alloy sheet of thepresent invention is explained below.

Sn Compounds:

In the present invention, the sheet after being produced (refined) has amicrostructure in which the average number density of Sn compounds whichhave a specific composition and a specific size and which are examinedwith an SEM having a magnification of 500 times and are identified withan X-ray spectrometer is specified and the amount of the boundariesbetween the Sn compounds and the aluminum matrix is specified.

The Sn compounds having a specific composition and a specific size areSn compounds (Sn-containing compounds) which contain both Mn and Fe orcontain either Mn or Fe and which have a content of Sn of 1.0 mass % orhigher and an equivalent circular diameter in the range of 0.3-20 μm.

The average number density of Sn compounds which satisfy suchrequirements is regulated so as to be in the range of 500-3,000counts/mm², preferably in the range of 500-2,000 counts/mm², therebyensuring a solute Sn amount necessary for enabling the solute Sn toexhibit the effect of inhibiting room-temperature age hardening.

Furthermore, the length of the boundaries between the Sn compounds,which satisfy those requirements, and the aluminum matrix is regulatedso as to be in the range of 3-20 /mm on average, preferably in the rangeof 3-10 /mm on average, in terms of a value obtained by dividing thetotal peripheral length of the Sn compounds by the area examined withthe SEM. By precipitating or crystallizing Sn compounds having thespecific composition and size so that the boundaries with the aluminummatrix are present in such a large amount, the boundaries betweenSn-free compounds, which reduce filiform corrosion resistance, and thematrix are diminished to improve the filiform corrosion resistance.

Average Number Density of the Sn Compounds:

In the case where the average number density of the Sn compounds havingthe specific composition and size is too high beyond 3,000 counts/mm², areduced solute Sn amount results, making it impossible to produce theSn's effect of inhibiting room-temperature age hardening. As a result,not only the room-temperature increase in strength cannot be inhibited,resulting in an increase in proof stress and a deterioration in hemworkability, but also Mg—Si precipitates are prone to be yielded in areduced amount during a BH treatment, resulting in a decrease in BHresponse.

Meanwhile, in the present invention, Sn is caused, to some degree, toprecipitate or crystallize out as compounds having the specificcomposition and size so that boundaries between these Sn compounds andthe matrix become large (long), in order to improve the filiformcorrosion resistance.

The present inventors investigated relationships between the addition ofSn and filiform corrosion resistance. As a result, the inventorsdiscovered that in the microstructure of an Al—Mg—Si alloy sheet, apeculiar phenomenon in which Sn added comes into coarse compounds torender them less apt to serve as filiform-corrosion starting pointsoccurs under certain production conditions.

The term “coarse compounds” herein means intermetallic compounds, suchas Al—Fe, Al—Fe—Mn, Al—Fe—Si, and Al—Fe—Mn—Si intermetallic compounds,which are relatively large and have an equivalent circular diameter ofsubmicrometers to tens of micrometers and that are yielded duringcasting, soaking, and hot rolling. In the cases when such coarsecompounds are present in an aluminum alloy, they have a nobler potentialthan the surrounding aluminum and serve as so-called cathode sites.

Consequently, the boundaries between these coarse compounds and thealuminum matrix have a large potential difference and are in the stateof being highly susceptible to corrosion. This corrosion phenomenonoccurs as filiform corrosion (rust extending in the form of threads) inthe case where the surface of the aluminum alloy sheet (panel) iscovered with a resinous coating film, as in the automotive panels.

In contrast, the inclusion of Sn in the coarse compounds reduces thepotential difference with the surrounding aluminum to render the coarsecompounds less apt to serve as cathode sites and less apt to serve asstarting points for filiform corrosion. Namely, the length of theboundaries between the Sn compounds and the aluminum matrix is regulatedso as to be not less than a certain value range and the boundariesbetween Sn-free compounds, which reduce filiform corrosion resistance,and the matrix are diminished. The filiform corrosion resistance canhence be improved.

Thus, formability and BH response, and satisfactory filiform corrosionresistance are combinedly exhibited.

Consequently, the specified average number density of the Sn compoundshaving the specific composition and size is a measure of the amount ofSn which has precipitated or crystallized out, for precipitating orcrystallizing Sn just in a certain amount (certain number density andcertain peripheral length) in order to improve the filiform corrosionresistance. In the case where the average number density of the specificSn compounds is too low and below 500 counts/mm², the specific Sncompounds themselves, which contain Mn and Fe, are not obtained and thefiliform corrosion resistance cannot be improved.

Sn Compounds Containing Mn and Fe:

In the sheet alloy composition, together with the Mn and Fe containedtherein, Sn forms Sn compounds having the specific composition and size.Hence, in the case where the sheet does not contain these Mn and Fe, Sncompounds themselves which have the specific composition and size arenot yielded. It is, however, noted that so long as Mn and Fe are presentin the Sn compounds in amounts on a level (range) detectable with theEDX which will be described later, the amounts thereof suffice, andthere is no need of quantitatively specifying the contents thereof inthe Sn compounds.

Sn Content and Size of the Sn Compounds:

Among Sn compounds, even when compounds containing Sn in a too smallamount, in which an Sn content is less than 1.0 mass %, or compoundshaving too small an equivalent circular diameter less than 0.3 μm arepresent so as to satisfy the average number density or the sufficientamount of the boundaries of the compounds, this cannot ensure a soluteSn amount, and the effect of improving formability, BH response,filiform corrosion resistance, etc. is low. Consequently, thesecompounds are excluded from the Sn compounds having the specificcomposition and size.

There is no particular upper limit on the Sn content of the specific Sncompounds. However, an upper limit thereof is about 10% by mass in viewof limitations in production. Meanwhile, in the case where the specificSn compounds are coarse compounds having an equivalent circular diameterexceeding 20 μm, they are causative of cracks, and cracks are prone tooccur during hot rolling, etc. in sheet production steps.

Length (Amount) of Boundaries of the Sn Compounds:

With respect to the state in which the Sn compounds having the specificcomposition and size are present in the sheet microstructure, in thecases when boundaries between these Sn compounds and the matrix are madelonger (present in a larger amount), the filiform corrosion resistanceis improved. In the case where the amount of the boundaries betweenthese Sn-containing compounds and the matrix is too small, the effect ofimproving filiform corrosion resistance is lessened. Specifically, inthe case where the length of the boundaries between these Sn compoundsand the aluminum matrix is less than 3/mm in terms of a value obtainedby dividing the total peripheral length of these compounds (total of theperipheral lengths of all the Sn compounds having the specificcomposition and size) by the area examined with the SEM, the boundariesbetween the Sn compounds and the matrix become short. Because of this,the boundaries between Sn-free compounds, which reduce the filiformcorrosion resistance, and the matrix are longer (present in an increasedamount) and the effect of improving filiform corrosion resistance islessened.

Meanwhile, in the case where the amount of the boundaries between the Sncompounds and the matrix is made too large beyond 20/mm, the numberdensity of the Sn-containing compounds is too high and a reduced soluteSn amount results, making it impossible to obtain a low proof stress andhigh BH response. Consequently, the amount of the boundaries between theSn-containing compounds and the matrix is regulated to 3-20/mm onaverage in terms of a value obtained by dividing the total peripherallength of these compounds by the area examined with the SEM. Morepreferably, it is in the range of 3-10/mm on average.

Examination of the Sn Compounds:

A measurement for determining the number density of compounds which havean equivalent circular diameter in the range of 0.3-20 μm and whichcontain 1.0 mass % or more Sn and further contain both Mn and Fe is madewith an SEM (scanning electron microscope) having a magnification of 500times. These Sn compounds having the specific composition and size areidentified with an X-ray spectrometer belonging to the SEM and aredistinguished from compounds which have an Sn content less than 1.0 mass% or which do not contain Mn or Fe. Furthermore, they are distinguished,with the SEM, also from compounds which do not satisfy the range ofsizes.

The measurement with the SEM is made with respect to ten portionsarbitrarily selected at a depth corresponding to ¼ the sheet thicknessdirection from a surface of a test sheet (ten specimens are collected).The number densities of Sn compounds having the specific composition andsize determined with respect to these specimens are averaged to obtainan average number density (counts/mm²). Specifically, as for across-section perpendicular to the sheet thickness direction of a testsheet which has just undergone a refining treatment, with respect to aplane which passes through arbitrarily selected points located at adepth corresponding to ¼ the sheet thickness direction from a surfaceand which is parallel with the sheet surface, an examination is madewith an SEM having a magnification of 500 times. Specimens are preparedin the following manner. Surfaces of ten sheet cross-section specimensobtained by sampling the above-described part are mechanically ground toremove a layer of about 0.25 mm from each sheet surface by themechanical grinding. The surfaces are then regulated by buffing toprepare the specimens. Next, the number of compounds having anequivalent circular diameter within the range shown above is countedwith an automatic analyzer while utilizing reflected-electron images,and a number density is calculated therefrom. The parts to be examinedare the polished specimen surfaces, and the examination region in eachspecimen is 240 μm×180 μm.

The X-ray spectrometer is well known as an analyzer based on energydispersive X-ray spectroscopy, is usually called EDX, and belongs to theSEM and is used for quantitative analysis for determining thecompositions of compounds each having an equivalent circular diameterwithin the above-described range. When determining the number ofcompounds each having an equivalent circular diameter within that range,the specific compounds are distinguished from other compounds by Sncontent and by whether Mn and Fe are substantially contained or not. TheSn compounds having the specific composition and size only areidentified. In the present invention, in the case where either Mn or Fecannot be detected in a compound with the X-ray spectrometer, this isregarded as a compound not containing Mn or Fe and as a compound otherthan the Sn compounds having the specific composition and size, as inthe case where the content of Sn is less than 1.0 mass %.

Furthermore, through analysis of reflected-electron images in the SEM,the total peripheral length (mm) of the Sn compounds having the specificcomposition and size is determined. This length is divided by the areaexamined with the SEM (field of view of the SEM; 240 μm×180 μm,converted to area in mm²), and the resultant values (mm/mm²) areaveraged with respect to the number of the specimens to determine thelength (/mm) of the boundaries with the aluminum matrix.

Differences With Conventional Art:

As described above, the Sn-containing 6000-series aluminum alloy sheetof the present invention differs from 6000-series aluminum alloy sheetsinto which Sn has been incorporated similarly (in the same amount), inboth microstructure and property because of the feature concerning thesolid-solution state of Sn and because of the feature in which thesolid-solution state is balanced with the Sn compounds which have beenprecipitated or crystallized. Specifically, differences in productionconditions regarding soaking treatment, etc. result in considerabledifferences in the present states, such as solute Sn amount, thecompositions and number density of Sn compounds, the amount ofboundaries with the matrix, etc.

In other words, under ordinary sheet production conditions (ordinaryprocesses), Sn is prone to precipitate as compounds and a considerablysmall solute amount results. In addition, these Sn compounds differ inthe composition and number density, and the boundaries with the matrixare present in a smaller amount. Because of this, even though Sn iscontained similarly (in the same amount), a microstructure which iseffective in inhibiting room-temperature age hardening on a high leveland in improving the BH response and hem workability and which givesexcellent filiform corrosion resistance as in the present inventioncannot always be obtained.

In a conventional Sn-containing 6000-series aluminum alloy sheets, suchan effect of Sn has been unable to be sufficiently exhibited. Thereasons for this are presumed to be because, although the formation ofsolid solution and the precipitation of Mg and Si, which are majorelements, have always been attracting attention hitherto, the existencestate of the solid-solution or precipitate of Sn, which merely is one ofselectively used additive elements, has been attracting littleattention. In the sheets produced by ordinary methods, the Sn is mainlypresent in the form of compounds formed by crystallization orprecipitation (hereinafter simply referred to as precipitation). Unlikethis, and because causing Sn to form a solid solution is difficult initself and the solid solution state of Sn is rare existence state, it ispresumed that the effect produced by Sn present in a solid-solutionstate has been less apt to be found out.

(Production Process)

Next, a process for producing the aluminum alloy sheet of the presentinvention is explained below. Production steps of the aluminum alloysheet of the present invention are themselves ordinary method or knownmethod. It may be produced by forming, by casting, a slab of an aluminumalloy having the 6000-series component composition, thereafterperforming a homogenizing heat treatment, hot rolling, and cold rollingto obtain a given sheet thickness, and then further performing arefining treatment such as a solution quenching treatment.

However, during the producing step, in order to make the sheet afterbeing produced (refined) have a microstructure in which the averagenumber density of Sn compounds having the specific Sn-containingcomposition and size is within the specified range and in which Sn hasformed a solid solution and the formation of solid solution of Sn isbalanced with the precipitation thereof, not only the average coolingrate during the casting is controlled but also use is made of preferredconditions specified for process annealing to be performed during thecold rolling, as will be described later. In the case where such processannealing conditions are not used, it is difficult to make the Sn form asolid solution.

In addition, a soaking treatment is conducted in two stages underspecific conditions in order to make the sheet after being produced(refined) have the microstructure in which the amount of the boundariesbetween the Sn compounds having the specific Sn-containing compositionand size and the aluminum matrix is within a specified range.

Melting and Casting Cooling Rate:

First, in melting and casting steps, an aluminum alloy melt that hasbeen melted and regulated so as to have a component composition withinthe 6000-series composition range is cast by a suitably selectedordinary melting and casting method, such as a continuous casting methodor a semi-continuous casting method (DC casting method). Here, from thestandpoint of causing the Sn to form a solid solution as specified inthe present invention, it is preferable that the average rate of cooingfrom the liquidus temperature to the solidus temperature during thecasting should be as high (quick) as possible at 30° C./min or greater.

In the case where such temperature (cooling rate) control in ahigh-temperature region during the casting is not performed, the coolingrate in this high-temperature region is inevitably low. Such a reducedaverage cooling rate in the high-temperature region results in a largeramount of coarsely yielded crystals in the temperature range of thehigh-temperature region and gives a slab having increased unevenness incrystal size or amount along the sheet width direction and thicknessdirection. As a result, it becomes highly probable that the Sn cannot bemade to form a solid solution within the ranges specified in the presentinvention.

Homogenizing Heat Treatment:

Next, the aluminum alloy slab obtained by casting is subjected to ahomogenizing heat treatment prior to hot rolling. The purpose of thishomogenizing heat treatment (soaking treatment) is to homogenize themicrostructure, that is, to eliminate segregation within the grains inthe microstructure of the slab.

In the present invention, however, the soaking treatment is conductedunder the following specific conditions in order that the sheet afterbeing produced (refined), after having undergone room-temperature agingafter the refining treatment, may have a microstructure in which theamount of the boundaries between Sn compounds having the specificcomposition and size and the aluminum matrix is within the specifiedrange.

In the first stage in the soaking treatment, holding is performed in therange of 400-500° C. for 1-10 hours. Sn compounds having the specificcomposition and size are thereby finely dispersed to regulate the numberdensity of these compounds and the amount of the boundaries with thealuminum matrix so as to be within the specified ranges. In the casewhere the soaking temperature is lower than 400° C. or the holding timeis less than 1 hour, it is difficult to finely disperse the Sn compoundshaving the specific Sn-containing composition and size to regulate theamount of the boundaries with the aluminum matrix to 3 /mm or larger onaverage in terms of a value obtained by dividing the total peripherallength of these compounds by the area examined with the SEM. Meanwhile,in the case where the holding time in the first stage exceeds 10 hours,the number density of the Sn compounds having the specific compositionand size becomes too high beyond 3,000 counts/mm², resulting in ashortage in the solute Sn amount which is necessary for inhibitingroom-temperature age hardening.

Subsequently, in the second stage in the soaking treatment in whichfurther heating is performed, holding is performed in the range of520-560° C. for 3 hours or longer. In this second stage in the soakingtreatment, Mg-Si-Sn compounds present as crystals in the slab are causedto form a solid solution to increase the solute Sn amount. In the casewhere the temperature in this second stage in the soaking treatment islower than 520° C. or the holding time therein is less than 3 hours, theformation of solid solution of the Mg—Si—Sn compounds present ascrystals in the slab is insufficient, resulting in a shortage in thesolute Sn amount which is necessary for inhibiting room-temperature agehardening. Meanwhile, in the case where the soaking temperature in thissecond stage exceeds 560° C., the slab suffers a fusion loss. Althoughthe holding time in the second stage may be long, there is no need ofprolonging it beyond 20 hours from the standpoints of productionefficiency and profitability.

So long as the holding time in the temperature range of 400-500° C. canbe set to 1-10 hours, the soaking treatment including two stages may beone in which holding is performed at a constant temperature or may be aheat treatment in which the temperature is gradually changed bytemperature raising, gradual cooling, etc., as described in the Exampleswhich will be described later. In short, the temperature may becontinuously changed by temperature raising, gradual cooling, etc. solong as the holding is performed in the temperature range of 400-500° C.for 1 hour or more and 10 hours or less.

Hot Rolling:

The hot rolling is constituted of a slab rough rolling step and a finishrolling step in accordance with the thickness of the sheet to be rolled.In the rough rolling step and finish rolling step, rolling mills such asa reverse type and a tandem type are suitably used.

In such conditions that the hot-rolling (rough-rolling) starttemperature exceeds the solidus temperature, burning occurs and, hence,the hot rolling itself is difficult to carry out. Meanwhile, in the casewhere the hot-rolling start temperature is lower than 350° C., the loadduring hot-rolling is too high, rendering the hot rolling itselfdifficult. Consequently, the hot-rolling start temperature is preferablyin the range of 350° C. to the solidus temperature, more preferably inthe range of 400° C. to the solidus temperature.

Annealing of Hot-Rolled Sheet:

Annealing (rough annealing) before cold rolling is not always necessaryfor the hot-rolled sheet. However, it may be performed in order tofurther improve properties such as formability by making the grainssmaller and optimizing the texture.

Cold Rolling:

In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolledsheet (including a coil) having a desired final sheet thickness.However, from the standpoint of making the grains even smaller, it isdesirable that the total cold rolling ratio should be 60% or greaterregardless of the number of passes.

Process Annealing:

It is preferable that before this cold rolling (after the hot rolling)or during the cold rolling (between passes), process annealing should beperformed to bring the Sn which has formed compounds in the precedingsteps including the hot rolling step into a solid-solution state. In theprocess annealing, the sheet is held for 0.1-10 seconds at a hightemperature of 480° C. or higher but not higher than the melting pointand then forcedly cooled (rapidly cooled) to room temperature at anaverage cooling rate of 3° C./sec or higher. In ordinary processes, theSn is prone to precipitate and the Sn which has once precipitated isconsiderably difficult to bring into a solid-solution state again. It isdifficult to cause the Sn to form a solid solution, as specified in thepresent invention, by merely performing the solution treatment whichwill be described later, and it is necessary to perform ahigh-temperature heat treatment by process annealing.

With respect to the conditions for this process annealing, in the casewhere the sheet temperature is lower than 480° C., an insufficientsolute Sn amount results. Meanwhile, in the case where the cooling afterthe annealing is not the forced cooling (rapid cooling) to roomtemperature at an average cooling rate of 3° C./sec or higher by aircooling, mist or water cooling, or the like, that is, in the case wherethe average cooling rate is less than 3° C./sec, the Sn which has onceformed a solid solution undesirably precipitates again to formcompounds.

Annealing under such conditions, including the rapid cooling, isimpossible with a batch type furnace, and a continuous heat treatmentfurnace is necessary in which the sheet is passed, while being unwound,through the furnace and wound up.

Solution and Quenching Treatments:

After the cold rolling, solution and quenching treatments are performed.The solution treatment and the quenching treatment may be heating andcooling which are performed on an ordinary continuous heat treatmentline, and are not particularly limited. However, from the standpoint ofobtaining a sufficient solid-solution amount of each element and becauseit is desirable that the grains of the microstructure of the sheetshould be finer, it is preferred to conduct the treatments under suchconditions that heating is performed at a heating rate of 5° C./sec orhigher to a solution treatment temperature of 520° C. or higher and nothigher than the melting temperature, followed by holding for 0-10seconds. The average rate of cooling from the solution treatmenttemperature to a quenching stop temperature is preferably regulated to3° C./sec or higher. In the case where the cooling rate is too low, thenumber density of the Sn compounds becomes too high, resulting in toosmall a solute Sn amount. It hence becomes difficult to satisfy a 0.2%proof stress during forming as low as 110 MPa or less, a hem workabilityof 2.0 or less, and a BH response through 185° C.×20 min of 100 MPa orgreater. In addition, Mg—Si compounds and the like are prone toprecipitate during the cooling, and they prone to serve as startingpoints for cracks during press forming or bending, resulting in adecrease in the formability. In order to secure that cooling rate, meanssuch as forced air cooling with fans or water cooling with mist or sprayor by immersion, etc. and conditions therefor are selected and used forthe quenching treatment.

The conditions for the solution and quenching treatments and for therough annealing after the hot rolling are akin to the conditions for theprocess annealing in temperature, etc. However, in the case where theprocess annealing is omitted or where it is performed but the variousconditions including a temperature of 520° C. or higher are notsatisfied, it is impossible to cause the Sn to form a solid solutionjust in the necessary amount or the specified amount by merelyconducting the solution and quenching treatments and the rough annealingafter the hot rolling.

Preliminary Aging Treatment (Reheating Treatment):

After such solution treatment, quenching and cooling to room temperatureare performed. Thereafter, the sheet is subjected to a preliminary agingtreatment (reheating treatment) as soon as possible in 1 hour (60minutes).

In the case where the room-temperature holding time from the end ofquenching to room temperature to initiation of the preliminary agingtreatment (initiation of heating) is too long and exceeds 1 hour,room-temperature age hardening proceeds, resulting in a decrease in BHresponse. Consequently, the shorter the room-temperature holding timeis, the better. The solution and quenching treatments and the reheatingtreatment may be consecutively performed so that there is substantiallyno pause therebetween, and there is no particular lower limit thereof.

With respect to the temperature and holding time in this preliminaryaging treatment, holding is preferably performed at a temperature in therange of 80-150° C. for 3 hours or more and 50 hours or less. In thistreatment, the holding in the temperature range of 80-150° C. may be aheat treatment in which temperature is constant within that temperaturerange or in which the temperature is gradually changed within thattemperature range by temperature raising or gradual cooling. In short,the temperature may be continuously changed by gradual cooling,temperature raising, etc., so long as the holding is performed in thetemperature range of 80-150° C. for 3 hours or more and 50 hours orless. Cooling to room temperature after the reheating treatment may bestanding to cool or may be conducted by forcedly cooling by using thecooling means used in the quenching, in order to heighten the efficiencyof the production.

Unless the preliminary aging treatment is performed under conditionswithin those preferred ranges, it is difficult to provide a sheet which,in forming into automotive panels, has a 0.2% proof stress reduced to110 MPa or less and which has a BH response of 100 MPa or greater.

The present invention will be explained below in more detail byreference to Examples. However, the present invention should not, ofcourse, be construed as being limited by the following Examples, and canbe suitably modified and performed as long as the modifications conformto the gist of the present invention described hereinabove andhereinafter. All such modifications are included in the technical rangeof the present invention.

EXAMPLES

Examples of the present invention are explained. 6000-series aluminumalloy sheets were individually produced so as to differ in the averagenumber density of Sn compounds having the specific composition and sizeand in the amount of the boundaries between the Sn compounds and thealuminum matrix, by changing the soaking treatment conditions or processannealing conditions. These sheets were held at room temperature for 30days after the production, and then evaluated for strength, BH response(bake hardenability), hem workability, and filiform corrosionresistance. The results thereof are shown in Table 2.

Specific conditions used for producing the aluminum alloy sheets were asfollows. Slabs of aluminum alloys respectively having the compositionsshown in Table 1 were commonly produced through casting by the DCcasting method. Here, the average rate of cooling from the liquidustemperature to the solidus temperature in the casting was set at 50°C./min in common with all the Examples. With respect to the indicationsof the contents of elements in Table 1, which show the compositions ofthe 6000-series aluminum alloy sheets of the Examples, the indicationsusing blanks as the values of elements each indicate that the contentthereof is below a detection limit and that the element is notcontained, i.e., 0%.

The slabs were each subjected to a soaking treatment under theconditions shown in Table 2, and hot rough rolling in each Example wasthen initiated at the temperature for the second stage. Thereafter, inthe succeeding finish rolling, hot rolling to a thickness of 2.5 mm isperformed to obtain hot-rolled sheets, in common with all the Examples.The hot-rolled sheets were subjected, in common with all the Examples,to process annealing with a continuous annealing furnace, duringcold-rolling passes (between passes), under various conditions as shownin Table 2. Thus, cold-rolled sheets (product sheets) having a thicknessof 1.0 mm were finally obtained.

Furthermore, these cold-rolled sheets were subjected to a solution heattreatment with a 560° C. niter furnace, hold for 10 seconds after atarget temperature had been reached, and then quenched by water coolingin which the average rate of cooling from the solution heat treatmenttemperature to the quenching stop temperature was 50° C./sec, in commonwith all the Examples. Immediately after this quenching, a preliminaryaging treatment was conducted in which holding is performed at 100° C.for 5 hours (after the holding, gradually cooling is performed at acooling rate of 0.6° C./hr).

From the sheets which had just undergone these refining treatments, testsheets (blanks) were cut out. As the microstructure of the test sheets,the average number density of Sn compounds having the composition andsize and the amount of the boundaries between the Sn compounds and thealuminum matrix were examined. Furthermore, test sheets (blanks) werecut out of the sheets which had been allowed to stand at roomtemperature for 30 days after the refining treatments, and examined forstrength (AS proof stress; 0.2% proof stress measured after 30-dayroom-temperature aging after the sheet production) and BH response. Theresults thereof are shown in Table 2.

(Microstructure of each test sheet)

With respect to each test sheet which had just undergone the refiningtreatments, among compounds containing Mn and Fe, the average numberdensity (counts/mm²) of compounds which had an Sn content of 1.0 mass %or higher and an equivalent circular diameter in the range of 0.3-20 μmwas determined by the measuring method in which an SEM having amagnification of 500 times and an X-ray spectrometer were used.

Furthermore, the lengths of the boundaries between the Sn compoundshaving the composition and size and the aluminum matrix were determinedas a value (/mm) obtained by dividing the total peripheral length of theSn compounds having the composition and size (total of the peripherallengths of all the Sn compounds having the composition and size) by thearea examined with the SEM, by the measuring method in which an SEMhaving a magnification of 500 times and an X-ray spectrometer were used.

(Tensile Test)

A tensile test was conducted in the following manner. No. 5 specimens(25 mm×50 mmGL×sheet thickness) according to JIS Z2201 were sampled fromeach test sheet which had been allowed to stand at room temperature for30 days after the refining treatments, and subjected to the tensile testat room temperature. Here, the tensile direction of each specimen wasset so as to be perpendicular to the rolling direction. The tensile ratewas set at 5 mm/min until the 0.2% proof stress and at 20 mm/min afterthe proof stress. The number N of examinations for mechanical propertywas 5, and an average value therefor was calculated. With respect to thespecimens to be examined for proof stress after BH, a 2% pre-strain as asimulation of sheet press forming was given to the specimens by thetensile tester, and the BH treatment was then performed.

With respect to properties of the sheets during forming after the 30-dayroom-temperature aging, the sheets having an As 0.2% proof stress (0.2%proof stress during forming) shown in Table 2 of 110 MPa or less and ahem workability, according to the criteria shown later in the Examples,of 2 or less were rated as acceptable regarding the formability ofsheets as materials for automotive outer panels.

(BH Response)

The test sheets were subjected to the 30-day room-temperature aging andthen to an artificial age hardening treatment of 185° C.×20 min, andwere thereafter examined for 0.2% proof stress (0.2% proof stress afterBH) through the tensile test, in common with the test sheets. The BHresponse of each test sheet was evaluated on the basis of the increaseamount in proof stress shown in Table 2 (difference between the 0.2%proof stress after BH and the As 0.2% proof stress). In the case wherethe increase amount in 0.2% proof stress was 100 MPa or greater, the BHresponse was regarded as acceptable.

(Hem Workability)

Hem workability was evaluated with respect to the test sheets which hadundergone the 30-day room-temperature standing. In the test,strip-shaped specimens having a width of 30 mm were used and subjectedto 90° bending at an inward bending radius of 1.0 mm with a down flange.Thereafter, an inner having a thickness of 1.0 mm was interposed, andthe specimen was subjected, in order, to pre-hem working in which thebent part was further bent inward to approximately 130° and flat-hemworking in which the bent part was further bent inward to 180° and theend portion was brought into close contact with the inner. The surfacestate, such as the occurrence of rough surface, a minute crack or alarge crack, of the bent part (edge bent part) of the flat hem wasvisually examined and visually evaluated on the basis of the followingcriteria. Ratings of 0 to 2 were acceptable.

-   0, no crack and no rough surface; 1, slight rough surface; 2, deep    rough surface; 3, minute surface crack; 4, linearly continued    surface crack; 5, fracture.

(Filiform Corrosion Resistance)

The test sheets which had undergone the room-temperature aging wereevaluated for filiform corrosion resistance. The test method used forthe evaluation was as follows. A sheet of 80×150 mm was cut out of eachtest sheet which had undergone the 3-day room-temperature aging, and wasimmersed in a sodium-carbonate-containing degreasing bath at 40° C. for2 minutes (with stirring with a stirrer) to degrease the test sheetsurfaces. Next, immersing was performed for 1 minute in azinc-containing surface-regulating bath having room temperature (withstirring with a stirrer), subsequently immersing was performed in a 35°C. zinc phosphate bath for 2 minutes to conduct a zinc phosphatetreatment, and further electrodeposition coating (thickness, 20 μm) wasperformed in accordance with an ordinary step for coating automotivemembers and then a 20-minutes baking treatment at 185° C. was performed.Thereafter, a cross cut incision having a length of 50 mm was made inthe coating film, and cycles each configured of 24-hour salt spray120-hour wetting (humidity, 85%; 40° C.)→24-hour air drying (roomtemperature) were performed for eight cycles. The width of the rust onone side of the cross cut part was measured as the length of filiformcorrosion.

The filiform corrosion resistance was evaluated in terms of the maximumwidth of the rust on one side of the cross cut part. The test sheet inwhich the maximum width was less than 1 mm was rated as ∘∘, that inwhich the maximum width was 1 mm or larger but less than 2 mm was ratedas ∘, that in which the maximum length was 2 mm or larger but less than3 mm was rated as Δ, and that in which the maximum length was 3 mm orlarger was rated as ×. The test sheets rated as ∘∘ and ∘ were regardedas excellent (acceptable) materials in terms of filiform corrosionresistance.

Invention Examples shown as Nos. 1 to 3, 9, 12, and 14 to 21 in Table 2are within the component composition range according to the presentinvention (alloys Nos. 1 to 11 in Table 1), and have been produced underconditions within the preferred ranges including those for soakingtreatment and process annealing. Because of this, these InventionExamples each satisfy both the average number density of Sn compoundshaving the composition and size specified in the present invention andthe amount of the boundaries between the Sn compounds and the aluminummatrix specified in the present invention, as shown in Table 2, and hasa satisfactory balance between the formation of solid solution of Sn andthe precipitation thereof.

As a result, as shown in Table 2, the Invention Examples each have anexcellent feature in which even after 30-day room-temperature agingafter the refining treatments, the As 0.2% proof stress during pressforming into automotive outer panels (before baking finish) is 110 MPaor less and the evaluation of hem workability is 0-2, and the automotiveouter panels can have an artificial-aging hardening amount (BHresponse), as measured under the bake finish conditions of 185° C.×20min, of 100 MPa or greater. They further have excellent filiformcorrosion resistance.

Meanwhile, in Comparative Examples 4 to 8, 10, 11, 13, 28, and 29, inwhich the soaking treatment conditions or the process annealingconditions were outside the preferred ranges although they use alloysNos. 1, 2, 3, 18, and 19 in Table 1, which are within the componentcomposition range according to the present invention, either the averagenumber density of Sn compounds having the composition and size specifiedin the present invention or the amount of the boundaries between the Sncompounds and the aluminum matrix specified in the present invention isoutside the specified range, as shown in Table 2. The formation of solidsolution of Sn has not been balanced with the precipitation thereof.

As a result, in each of these Comparative Examples, the proof stressduring press forming into automotive outer panels after 30-dayroom-temperature aging after the refining treatments is too high beyond110 MPa, the BH response is as low as below 100 MPa, or the filiformcorrosion resistance is poor, as shown in Table 2.

In Comparative Examples 4, 6 and 13, the holding time in the first stagein the soaking treatment was too short or the first stage in the soakingtreatment was not performed. Because of this, the average number densityof Sn compounds having the composition and size described above is toolow, the amount of the boundaries between the Sn compounds and thematrix is less than 3/mm, and the filiform corrosion resistance is poor.

In Comparative Examples 5, 7 and 10, the holding time in the first stagein the soaking treatment was too long or the soaking treatmenttemperature in the second stage was too low. Because of this, Sncompounds have been yielded in too large an amount and a sufficientsolute Sn amount cannot be ensured. Consequently, AS proof stress ishigh and proof stress increase amount is small. In addition, the processannealing was not performed in Comparative Example 7, and the rate ofthe cooling after the process annealing in Comparative Example 10 wastoo low.

In Comparative Examples 8 and 11, the process annealing temperature wastoo low. Because of this, Sn compounds have been yielded in too large anamount and a sufficient solute Sn amount cannot be ensured.Consequently, AS proof stress is too high and proof stress increaseamount is small.

In Comparative Examples 28 and 29, use was made of alloys Nos. 18 and 19in Table 1, which are within the component composition range accordingto the present invention. However, the process annealing was notperformed, or the rate of the cooling after the process annealing wastoo low. Because of this, Sn compounds have been yielded in too large anamount and a sufficient solute Sn amount cannot be ensured.Consequently, AS proof stress is too high and proof stress increaseamount is small.

Comparative Examples 22 to 27 and 30 to 32 in Table 2 have been producedunder the preferred condition ranges, but alloys Nos. 12 to 17 and 20 to22 in Table 1 were used therefor. Hence, the content of any of Mg, Siand Sn, which are essential elements, is outside the range according tothe present invention. Because of this, in each of Comparative Examples22 to 27 and 30 to 32, the proof stress during press framing after30-day room-temperature aging after the refining treatment is too highbeyond 110 MPa, the BH response is as low as below 100 MPa, or thefiliform corrosion resistance is poor, as shown in Table 2.

Comparative Example 22 is alloy 12 of Table 1, in which the Si contentis too low.

Comparative Example 23 is alloy 13 of Table 1, in which the Si contentis too high.

Comparative Example 24 is alloy 14 of Table 1, in which the Sn contentis too low.

Comparative Example 25 is alloy 15 of Table 1, in which the content ofSn is too high. Because of this, cracks were generated during the hotrolling, making the production of a hot-rolled sheet itself impossible.

Comparative Example 26 is alloy 16 of Table 1, in which the Fe contentis too high.

Comparative Example 27 is alloy 17 of Table 1, in which the Mn contentis too high.

Comparative Example 30 is alloy 20 of Table 1, in which the Fe and Mncontents are too low. Comparative Example 31 is alloy 21 of Table 1, inwhich the Mg content is too low.

Comparative Example 32 is alloy 22 of Table 1, in which the Mg contentis too high.

Those results of the Examples establish the critical significance oreffects of the composition specified in the present invention and thefeature of balancing the formation of solid solution of Sn with theprecipitation thereof or of the preferred production conditions, withrespect to combinedly achieving strength after room-temperature aging,formability, in particular, hem workability, BFI response, and filiformcorrosion resistance of Sn-containing 6000-series aluminum alloy sheets.

TABLE 1 Alloy Chemical components of Al—Mg—Si alloy sheet (mass %;remainder, Al) No. Mg Si Sn Fe Mn Cr Zr V Ti Cu Zn Ag 1 0.45 1.02 0.0360.19 ower 2 0.42 0.82 0.025 0.21 0.41 3 0.39 1.18 0.058 0.20 0.04 0.05 40.55 0.83 0.041 0.20 0.09 0.22 5 0.36 1.23 0.084 0.21 0.12 0.20 6 0.541.31 0.053 0.22 0.09 0.05 0.05 7 0.55 0.79 0.197 0.07 0.10 0.16 0.01 80.45 0.93 0.040 0.70 0.10 0.03 0.60 9 0.64 1.15 0.027 0.22 0.09 0.12 100.47 1.23 0.055 0.19 0.12 0.30 11 0.71 0.72 0.007 0.19 0.12 0.10 0.10 120.82 0.47 0.042 0.18 0.10 13 0.40 2.11 0.042 0.20 0.09 14 0.59 1.000.002 0.21 0.12 15 0.61 1.12 0.452 0.20 0.10 16 0.40 0.76 0.053 1.270.11 17 0.51 1.00 0.049 0.19 0.78 18 0.62 1.03 0.052 0.21 0.12 19 0.730.81 0.053 0.22 0.09 20 0.55 1.26 0.051 0.01 0.01 21 0.25 1.01 0.0430.15 0.16 22 1.23 1.04 0.048 0.15 0.14

TABLE 2 Process annealing between Soaking treatment cold-rolling passesFirst Second (continuous annealing) stage First stage Second AverageAlloy Temper- stage Temper- stage Temper- cooling No. in ature Timeature Time ature rate Classification No. Table 1 ° C. hr ° C. hr ° C. ×5 sec ° C./sec Inv. Ex 1 1 450 3 550 3 480 5 Inv. Ex. 2 1 420 8 550 3490 10 Inv. Ex. 3 1 480 2 550 3 510 50 Comp. Ex. 4 1 450 0.5 550 3 52050 Comp. Ex. 5 1 450 20 550 3 520 50 Comp. Ex. 6 1 — — 550 3 520 50Comp. Ex. 7 1 450 3 450 3 — — Comp. Ex. 8 1 450 3 550 3 450 50 Inv. Ex.9 2 450 5 550 3 520 50 Comp. Ex. 10 2 450 3 450 3 510 1 Comp. Ex. 11 2450 3 550 3 450 50 Inv. Ex. 12 3 450 3 550 3 510 50 Comp. Ex. 13 3 — —550 3 520 50 Inv. Ex 14 4 450 3 550 3 520 50 Inv. Ex. 15 5 450 3 550 3520 50 Inv. Ex. 16 6 450 3 550 3 520 50 Inv. Ex 17 7 450 3 550 3 520 50Inv. Ex. 18 8 450 3 550 3 520 50 Inv. Ex 19 9 450 3 550 3 520 50 Inv.Ex. 20 10 450 3 550 3 520 50 Inv. Ex. 21 11 450 3 550 3 520 50 Comp. Ex.22 12 450 3 550 3 520 50 Comp. Ex. 23 13 450 3 550 3 520 50 Comp. Ex. 2414 450 3 550 3 520 50 Comp. Ex. 25 15 450 3 550 3 cracks in hot rollingComp. Ex. 26 16 450 3 550 3 520 50 Comp. Ex. 27 17 450 3 550 3 520 50Comp. Ex. 28 18 450 3 550 3 — — Comp. Ex. 29 19 450 3 550 3 500 1 Comp.Ex. 30 20 450 3 550 3 520 50 Comp. Ex. 31 21 450 3 550 3 520 50 Comp.Ex. 32 22 450 3 550 3 520 50 Microstructure of aluminum alloy sheetafter refining Properties of aluminum alloy sheet after Average Lengthof 30-day room-temperature holding number boundaries As Increase densityof between Sn As 0.2% amount Sn compounds tensile proof in proof HemFiliform Classifi- compounds and matrix/ strength stress stress work-corrosion cation No. counts/mm² mm MPa MPa MPa ability resistance Inv.Ex 1 830 4.2 218 93 122 1 ∘ Inv. Ex. 2 910 5.6 214 90 126 1 ∘ Inv. Ex. 3660 3.5 219 92 125 1 ∘ Comp. Ex. 4 380 2.8 212 88 131 1 Δ Comp. Ex. 53860 13.3 229 112 92 2 ∘∘ Comp. Ex. 6 320 2.1 221 95 129 1 Δ Comp. Ex. 74820 22.2 231 114 88 2 ∘∘ Comp. Ex. 8 4010 10.6 239 118 93 2 ∘∘ Inv. Ex.9 2850 9.0 211 94 108 2 ∘∘ Comp. Ex. 10 5470 23.9 224 106 73 2 ∘∘ Comp.Ex. 11 3820 13.8 248 122 87 2 ∘∘ Inv. Ex. 12 680 3.6 219 96 118 1 ∘Comp. Ex. 13 280 1.8 215 91 124 1 Δ Inv. Ex 14 2630 10.3 226 102 112 2∘∘ Inv. Ex. 15 2710 8.7 210 94 108 2 ∘ Inv. Ex. 16 1020 5.0 231 106 1171 ∘ Inv. Ex 17 1090 5.5 195 80 129 1 ∘ Inv. Ex. 18 2720 11.7 214 91 1132 ∘ Inv. Ex 19 830 4.3 230 105 133 1 ∘ Inv. Ex. 20 1460 6.8 224 92 124 1∘ Inv. Ex. 21 760 3.9 228 106 120 1 ∘ Comp. Ex. 22 260 1.4 185 68 61 0∘∘ Comp. Ex. 23 7570 30.6 227 108 103 4 x Comp. Ex. 24 80 0.9 253 128 863 Δ Comp. Ex. 25 cracks in hot rolling Comp. Ex. 26 8240 22.5 214 95 834 x Comp. Ex. 27 8830 25.1 220 102 82 4 x Comp. Ex. 28 3250 13.4 231 11791 2 ∘ Comp. Ex. 29 3380 16.1 222 113 84 2 ∘ Comp. Ex. 30 20 0.4 198 87131 1 Δ Comp. Ex. 31 760 3.3 166 65 53 1 ∘∘ Comp. Ex. 32 1620 7.0 262141 117 3 ∘

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope of the presentinvention.

The present application is based on a Japanese patent application filedon Aug. 27, 2014 (Application No. 2014-173277), the whole thereof beingincorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provideSn-containing 6000-series aluminum alloy sheets which satisfy therequirements as automotive outer panels, concerning formability and BHresponse after room-temperature aging, and which further have improvedfiliform corrosion resistance. As a result, the 6000-series aluminumalloy sheets are usable in extended applications, especially asautomotive outer panels.

1. An aluminum alloy sheet which is an Al—Mg—Si alloy sheet comprising,in terms of mass %: 0.3-1.0% of Mg, 0.5-1.5% of Si, 0.005-0.2% of Sn,0.02-1.0% of Fe, and 0.02-0.6% of Mn, with the remainder being Al andunavoidable impurities, the aluminum alloy sheet having a microstructurewherein: among compounds as examined with an SEM with a magnification of500 times and identified with an X-ray spectrometer, an Sn compoundcomprising Mn and Fe, having an Sn content of 1.0 mass % or higher, andhaving an equivalent circular diameter in a range of 0.3-20 has anaverage number density in a range of 500-3,000 counts/mm²; and aboundary between the Sn compound and an aluminum matrix has a length ina range of 3-20 /mm on average in terms of a value obtained by dividinga total peripheral length of the Sn compound by an area as examined withthe SEM.
 2. The aluminum alloy sheet according to claim 1, furthercomprising at least one kind of ingredient selected from the groupconsisting of more than 0% and 0.4% or less of Cr, more than 0% and 0.3%or less of Zr, more than 0% and 0.3% or less of V, more than 0% and 0.1%or less of Ti, more than 0% and 0.4% or less of Cu, more than 0% and0.2% or less of Ag, and more than 0% and 1.0% or less of Zn, in terms ofmass %.
 3. An Al—Mg—Si alloy comprising, in terms of mass %, 0.3-1.0% ofMg, 0.5-1.5% of Si, 0.005-0.2% of Sn, 0.02-1.0% of Fe, and 0.02-0.6% ofMn, with the remainder being Al and unavoidable impurities.
 4. TheAl—Mg—Si alloy of claim 3, further comprising at least one kind ofingredient selected from the group consisting of more than 0% and 0.4%or less of Cr, more than 0% and 0.3% or less of Zr, more than 0% and0.3% or less of V, more than 0% and 0.1% or less of Ti, more than 0% and0.4% or less of Cu, more than 0% and 0.2% or less of Ag, and more than0% and 1.0% or less of Zn, in terms of mass %.
 5. A sheet comprising theAl—Mg—Si alloy of claim
 3. 6. A sheet comprising the Al—Mg—Si alloy ofclaim
 4. 7. The sheet according to claim 5 that comprises an aluminummatrix containing a Sn compound comprising Sn, Mn and Fe, wherein thecontent of Sn in the compound is 1.0% by mass or higher, wherein the Sncompound in the aluminum matrix has an equivalent circular diameterranging from 0.3 to 20 μm and the Sn compound has an average densityranging from 500 to 3,000 counts/mm², and wherein a boundary between theSn compound and the aluminum matrix ranges in average length rangingfrom 3 to 20/mm and has a length in a range of 3-20 /mm on average interms of a value obtained by dividing a total peripheral length of theSn compound by an area as examined with the SEM.
 8. The sheet accordingto claim 6 that comprises an aluminum matrix containing a Sn compoundcomprising Sn, Mn and Fe, wherein the content of Sn in the compound is1.0% by mass or higher, wherein the Sn compound in the aluminum matrixhas an equivalent circular diameter ranging from 0.3 to 20 μm and the Sncompound has an average density ranging from 500 to 3,000 counts/mm²,and wherein a boundary between the Sn compound and the aluminum matrixranges in average length ranging from 3 to 20/mm and has a length in arange of 3-20 /mm on average in terms of a value obtained by dividing atotal peripheral length of the Sn compound by an area as examined withthe SEM.